This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-215214, filed Jul. 29, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a noise reduction method for a radiation detector for converting radiation such as X-rays into an electrical signal in accordance with its intensity.
A planar type radiation detector has a plurality of pixels arrayed in the form of a matrix. Each pixel has a photoelectric conversion element and pixel electrode. When radiation is incident on the photoelectric conversion element, the photoelectric conversion element generates charge in an amount corresponding to the incident intensity. This charge is stored in a capacitor through the pixel electrode. The stored charge is read out from the capacitor through a readouting section.
FIG. 1 is a view showing a typical example of the arrangement of a conventional radiation detector. Referring to FIG. 1, a plurality of pixel electrodes 71 arranged in the form of a two-dimensional matrix acquire charge generated in photoelectric conversion films in accordance with the intensity of radiation that has passed through an object and struck on the films. A capacitor used as a charge storage element for storing the acquired charge (pixel charge) is connected to each pixel electrode 71. The pixel charge stored in the each capacitor is read out through a thin-film transistor (TFT) 72.
A gate line driver 74 selectively applies a gate voltage to a gate line 73 to turned on the gates of the TFTs 72. A plurality of TFTs 72 connected to the selected gate line 73 are simultaneously turned on. Capacitor charges on the same row are read out as electrical signals (to be referred to as detection signals hereinafter) to amplifiers 76 through signal lines 75. Thereafter, the amplified detection signals are sequentially sent to an A/D converter 78 through a multiplexer 77.
Note that a layer on which the above gate lines 73 are laid to be parallel to each other and a layer on which the signal lines 75 are laid to be parallel to each other are overlaid in a direction perpendicular to the drawing surface of FIG. 1 through an insulating layer. That is, the gate lines 73 and signal lines 75 are formed on different layers so as not to be short-circuited.
In general, such radiation detectors digitize radiation images and hence are very advantageous in terms of, for example, transmission, storage, and search of radiation images as compared with conventional radiation photographic films. It is expected that these detectors will become more popular. The above radiation detector designed to directly digitize radiation has the merit of easily obtaining digital images as compared with a conventional film digitizer scheme and the like.
In the above conventional planar type radiation detector, noise components that become xe2x80x9cdisturbance factorsxe2x80x9d are superimposed on detection signals. In general, this makes it difficult to obtain accurate object information. In this case, as the xe2x80x9cdisturbance factorsxe2x80x9d, the following two factors are conceivable.
The first factor is associated with a xe2x80x9cdark imagexe2x80x9d. It is known that in the photoelectric conversion film provided for the pixel electrode 71, a current generally called a xe2x80x9cdark currentxe2x80x9d is generated owing to, for example, the random thermal agitation of free electrons even when no radiation is incident. In addition, in general, an offset noise voltage is always observed in the amplifier 76. These dark currents and offset noise voltages are finally constructed into images through the signal lines 75 and amplifiers 76, thereby forming xe2x80x9cdark imagesxe2x80x9d.
For this reason, an image constructed on the basis of a detection signal having undergone no correction processing is the one obtained by superimposing the above dark image on a normal desired image. In order to obtain an accurate image, therefore, information associated with the dark image must be subtracted from the overall information.
Such an inconvenience has been recognized in the conventional scheme as well, and hence a method of acquiring correction information associated with a dark image in advance and subtracting it from detected information has been proposed. In general, however, the above dark current and offset noise voltage vary with temperatures, and the dark image changes accordingly. For this reason, the above method is not very effective. That is, since correction information is fixed, this method cannot cope with an actual condition (temperature) that incessantly changes over the operation time of the radiation detector and the like.
In addition, dark images change depending on the method of driving the gate lines 73 and TFTs 72. This is because the TFT 72 is not an ideal switching element, which has finite resistances in both ON and OFF states. This characteristic poses the following problem. A current or charge information that should be obtained in accordance with an array driving sequence dissipates or unnecessary components are added thereto. Consider a general sequence as a driving sequence, in which the ith gate line 73 is driven (the TFTs 72 on this line are turned on) to extract charge information from the corresponding pixel electrodes 71, and then the (i+1)th gate line 73 is driven at the same time when the driving of the ith gate line 73 is stopped (TFTS 72 are turned off). Current dissipation and addition may occur in the following two cases.
In the first case, a current dissipates as the TFTs 72 on the ith line are turned on and off. This is because the potential of the signal line 75 is equal to that of the pixel electrode 71 when the TFT 72 is turned on, but the potential of the pixel electrode 71 drops when the TFT 72 is turned off. This will be described in detail below. As shown in FIG. 2A, when the TFT 72 is ON, the charge given by
Q=Cgsxc2x7Von
is stored in a capacitor (capacitance Cgs) assumed to be formed on the source side of the TFT 72. At this time, as obvious from FIGS. 1 and 2A, the node of a capacitor (capacitance Cpx) provided for the pixel electrode 71 and the capacitor Cgs is grounded (GND level). Considering that the node is disconnected from the GND level when the TFT 72 is turned off, and charge Q stored in the capacitor Cgs is distributed into the capacitors Cpx and Cgs, the following relation is established (refer to FIG. 2B):
xe2x88x92Q=xe2x88x92Qxe2x80x2+Qxe2x80x3
Voffxe2x88x92Qxe2x80x2/Cgsxe2x88x92Qxe2x80x3/Cpx=0
where Qxe2x80x2 is the charge of the capacitor Cgs, and Qxe2x80x3 is the charge of the capacitor Cpx. From the above three equations, the charge Qxe2x80x3 stored in the capacitor Cpx can be given by
Qxe2x80x3=xe2x88x92Cxe2x80x2xc2x7(Vonxe2x88x92Voff)
where Cxe2x80x2=Cpxxc2x7Cgs/(Cpx+Cgs).
In this state, the potential of the capacitor Cpx, i.e., a potential V of the pixel electrode 71, is given by
V=Qxe2x80x3/Cpx=xe2x88x92(Cxe2x80x2/Cpx)xc2x7(Vonxe2x88x92Voff)
and Von greater than Voff generally holds. Therefore, V less than 0. That is, when the TFT 72 is turned off, the potential drops.
As described above, when the potential of the pixel electrode 71 drops as the TFT 72 on the ith line is turned on/off, a voltage is applied between the source and drain of the TFT 72. Obviously, a current is generated in this portion. As a consequence, excess charge is stored in the pixel electrode 71. When the ith gate line 73 is driven again, this excess charge information is additionally read out. This makes it impossible to obtain accurate information. Note that such charge addition will be referred to as xe2x80x9cfirst type offset noisexe2x80x9d hereinafter for the sake of convenience.
In the second case, assuming that the (i+1)th gate line 73 is driven, all the remaining gate lines 73, as well as the ith gate line 73, are not driven. In this state, it is expected that all the charge information stored in the pixel electrodes 71 connected to the (i+1)th gate line 73 will reach the multiplexer 77 through the signal lines 75. However, since the signal lines 75 are connected to all the gate lines 73 other than the (i+1)th gate line 73, currents flow to them. The currents flowing to the gate lines 73 other than the (i+1th) gate line 73 are stored as excess charges in the pixel electrodes 71. Therefore, when charge information is read out, unnecessary information is added to the charge information, and hence accurate information cannot be obtained as in the above case. Note that such charge addition will be referred to as xe2x80x9csecond type offset noisexe2x80x9d hereinafter for the sake of convenience. Obviously, the above description about the (i+1)th gate line applies to a case wherein any of the remaining gate lines is driven. That is, every time a gate line is driven, the charge that causes second type offset noise generally increases.
As described above, information (detection signal) associated with an image obtained through the multiplexer 77 is obtained with first type offset noise and second type offset noise being added thereto. As is obvious from the above description, these xe2x80x9camountsxe2x80x9d depend on the gate line driving method and the like, and hence are not always constant (this will be described in detail in xe2x80x9cDETAILED DESCRIPTION OF THE INVENTIONxe2x80x9d). The sum of first type offset noise and second type offset noise singly forms a dark image, and influences a dark image that originates from the above dark current and offset noise. As a consequence, this causes incessant variations in dark image. When an accurate image is to be obtained, a dark image must be corrected in consideration of such variations, and the variations must be coped with.
The second factor as a xe2x80x9cdisturbance factorxe2x80x9d is associated with the production of a noise component. In the above description, the gate lines 73 and signal lines 75 are formed on the different layers through the insulating layer in the planar type radiation detector. In this arrangement, each gate line 73 and the corresponding signal line 75 inevitably have an intersection. This intersection serves as a capacitance (capacitor). In practice, if the voltage of each signal line 75 is constant, charge is stored in the intersection capacitance owing to the voltage of the gate line 73.
In such a case, assuming that an ideal state is set, for example, the voltage of each gate line 73 is constant. In this case, the amount of charge stored in the intersection is constant, and no significant problem arises. In practice, however, the voltage of each gate line 73 fluctuates, and the charge stored in the intersection capacitance varies over time. Such a change in charge is transferred to the signal line 75 and becomes a xe2x80x9cnoise componentxe2x80x9d in the detection signal. As this noise component, the same voltage variation is added to the pixel electrodes 71 connected to the same gate line 73. As a consequence, identical noise components are uniquely added to the respective rows of the matrix.
Since this noise appears as an artifact in the form of a transverse line on an image in correspondence with a gate line, the noise is generally called xe2x80x9cline artifact noisexe2x80x9d in some cases. In this specification, to indicate noise in the above form, the term xe2x80x9cline artifact noisexe2x80x9d is used, and the amount of this noise is represented by n(i). In this case, the subscript xe2x80x9cixe2x80x9d indicates the ith row of the matrix.
A means for correcting the line artifact noise n(i) has already been proposed in Jpn. Pat. Appln. KOKAI Publication No. 9-197053. According to this proposal, the line artifact noise n(i) (this noise is called xe2x80x9ccommon mode noisexe2x80x9d instead of xe2x80x9cline artifact noisexe2x80x9d) is corrected by subtracting an output from a shield pixel which is shielded against radiation from an output from a non-shield pixel on which radiation is incident as in a normal case.
According to this means, an output signal which is expected to contain noise added to an output from a shield pixel, e.g., noise added in a subsequent circuit after readout operation, is subtracted from an output signal from a non-shield pixel without correcting the noise. There is therefore a chance that noise may be increased to result in difficulty in obtaining correct information.
As described above, all the disturbance factors described above become hindrances to the acquisition of accurate images. Therefore, correction must be properly executed in accordance with the characteristics of the above disturbance factors instead of using the obtained detection signal without any change.
In this case, in accordance with a finding that indicates a specific portion in the radiation detector in which the above disturbance factor is caused, correction may be performed by using gain noise (undesired amplification factor), offset noise (undesired offset amount) associated with xe2x80x9conlyxe2x80x9d this portion, or the like. This operation has greater significance in obtaining an accurate image. Consider, for example, the above dark images, i.e., the dark image due to the dark current in the photoelectric conversion film and the dark image due to the amplifier 76. The former is produced in the radiation detecting section, and the latter is produced in the readouting section. That is, they are produced in different portions. In addition, the radiation detector and readouting section respectively have unique gain noise and offset noise amounts. If, therefore, these values can be separately obtained from the respective portions, the acquisition of these values can contribute to proper correction. In the prior art, however, since the amplifier input capacitance of each signal line 75 based on an output form the radiation detecting section is large, the S/N ratio decreases. It is therefore taken for granted that gain noise or offset noise associated with only the radiation detecting section or readouting section is difficult to acquire.
It is an object of the present invention to provide a noise reduction method and radiation detector which can efficiently reduce not only noise due to a dark current and offset noise but also line artifact noise due to variations in voltage applied to each gate line over time.
According to the present invention, there is provided a radiation detector noise reduction method comprising the steps of detecting incident radiation with a radiation detecting section, the radiation detecting section having a plurality of pixels arrayed in the form of a matrix, reading out the detection signal from the radiation detecting section through a readouting section, and correcting the readout detection signal with a correction section, wherein the correction step includes the first sub-step of correcting the detection signal on the basis of a correction value corresponding to noise originating in the radiation detecting section, and the second sub-step of correcting the detection signal on the basis of a correction value corresponding to noise originating in the readouting section, the second sub-step being executed before the first sub-step.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.